The present invention relates to a mould structure for producing metal casts obtained by the solidification of molten metal, and to a casting method which uses such a structure.
In particular, the invention relates to the production of light metal alloy casts such as, for example, the crankcase of an internal combustion engine.
The mould structure one the invention is intended to be used in a low-pressure castling method, the general characteristics of which are well known in the art.
According to this method, the molten metal is fed into the mould through an inlet formed in the bottom of the mould cavity and fills this cavity from below, the molten metal is fed from a furnace positioned beneath the mould, either by means of a pump or by pressurizing the furnace so that the molten metal flows up inside the mould.
Low-pressure casting methods of the type described above are described, for example, in GB 1028736 and in EP-B-0 183 761.
U.S. Pat. No. 4,733,714 describes an improved method of low-pressure a primary source through an inlet below the top of the mould cavity the mould is then turned over in order to prevent the molten metal from flowing back from the mould cavity towards the primary source, and also to enable metal to flow into the cavity from a secondary source, constituted by a feed-head full of molten metal, keeping the mould cavity in continuous communication with the primary source while the mould is turned over. The mould cavity is then disconnected from the primary source and moved to a cooling station where the molten metalxe2x80x94contained in the secondary sourcexe2x80x94can flow into the mould while the metal solidifies.
The method described above means that the casting operation is largely independent of the time required for the casting to solidify, thus improving the productivity of the casting station.
EP-B-0 557 374 describes a similar casting method to that described in U.S. Pat. No. 4,733,714, in which the mould structure has a primary inlet and a secondary system for feeding molten liquid metal into the mould cavity while the metal is solidifying, thereby;compensating for shrinkage, and has a large-surface heat extraction element which defines a portion of the mould cavity won the opposite side from the secondary feeding system and adjacent the primary inlet.
According to the above description, the heat extraction element, which is intended to be in contact with outside means for extracting heat, plays a useful role in directing the solidification of the casting. In a preferred embodiment, the mould structure has means for sealing and insulating the mould cavity from the primary source when a substantial portion of the metal fed into the mould cavity is still in a liquid state.
The primary object of the present invention is to provide a new mould structure for lowxe2x80x94pressure casting whichxe2x80x94the weight and dimensions of the desired casting being equal makesxe2x80x94makes it possible to reduce the overall weight of the mould and the production costs thereof, while at the same time making it possible to achieve optimum solidification of the casting, avoiding the production of any inaccurate structures or unwelcome porosity.
These and other objects, which will become more apparent later, are achieved according to the invention by providing a mould structure of a type which includes a plurality of cores defining a mould cavity and a primary inlet for feeding the molten metal into the bottom region of the mould cavity through the walls of one of the said cores, characterised in that it includes:
a metal containment structure, open at the top, with a bottom wall and side walls having a plurality of reference elements with surface portions which are complementary with surface portions of the said cores, the said cores being assembled inside the containment structure so as to engage each other and the said reference elements, and being shaped so as to define interstices between their surfaces facing outwardly of the mould cavity and the walls of the containment structure, and
at least one covering element of a material which has a high heat conductivity, supported by the side walls of the containment structure and by some of the said cores, for closing the top of the mould cavity.
A further object of the invention is to provide a low-pressure casting method, using the mould structure described above.
One particular advantage of the mould structure of the invention consists in the fact that it makes it possible to use shaped cores of bound siliceous sand which are very thin and thus light and economical, together with metal cores.